Field of the Invention
The invention relates to an optical WDM transmission network.
Description of Related Art
A communication system comprising an optical line terminal (OLT), also referred to as head end, central node or hub, which is connected to a plurality of optical network terminals (ONTs) or optical network units (ONUs) by a remote node, often referred to as remote node (RN), is commonly designated as passive optical network (PON). In such a PON, the OLT realizes an interface between an access network and a backbone network to which the OLT is connected. The traffic of signals between the OLT and the remote node is realized in form of optical WDM transmission signals (downstream and upstream) comprising a given number of optical channel signals. Each optical channel signal is created at a dedicated (channel) wavelength out of a predetermined number of channel wavelengths. The so-called OLT traffic comprises all point-to-point transmission links, each point-to-point transmission link being realized by a pair of downstream and upstream channel signals at dedicated wavelengths. The downstream WDM signal is split at the remote node into the single channel signals that are transmitted to the respective ONUs. The upstream channel signals created in the ONUs are combined at the remote node to form the upstream WDM signal that is transmitted to the OLT.
Such a PON has the advantage that the connection network between the OLT and the ONU is a fully passive network that can be realized and maintained at low cost. Further, such a passive connection network does not require any power supply.
It is noted that, within the present description, the terms OLT, RN, ONU and PON shall be understood in their broadest sense and are not restricted to the specific term of a passive optical network as used in the literature. Rather, the terms OLT, RN and ONU designate the general devices realizing a central network node configured to transmit and receive a respective downstream and upstream optical WDM signal, a network node configured to split the downstream optical WDM signal and combine the upstream optical channel signals (constituting the OLT traffic) and a network node configured to receive and transmit respective channel signals, respectively.
In such communication networks, it might be advantageous to allow for direct communication between the ONUs via the remote node. Such direct transmission links have a generally lower latency than corresponding transmission links which are realized through signals that are transmitted the whole way from an ONU to the OLT and from the OLT to the selected further ONU. As described in EP 2 475 121 A1, such a network realizing OLT traffic and simultaneously allowing for realizing a star structure by providing star links (each star link realized between two selected ONUs via the remote node) may be used to connect a plurality of base stations to a central switching unit in a mobile backhaul access network and to simultaneously allow for direct connections between the base stations via an optical multiplexer/demultiplexer, which realizes a remote node in the above-explained terminology. The remote node is realized by a cyclic N×N arrayed waveguide grating (AWG) wherein the ONUs are connected by a single optical fiber to a dedicated back port of the N×N AWG and the central node or OLT is connected to a selected front port of the N×N AWG. In order to provide star links between one or more selected pairs of ONUs, the remaining front ports of the N×N AWG are connected to the input ports of a first optical splitter/combiner device the common output port of which is connected to the front port of the N×N AWG to which the central node or OLT is connected. For this purpose, a second splitter/combiner device may be used wherein the optical fiber or path connecting the OLT is connected to a first splitting port of the splitter/combiner device and the common output port of the first splitter/combiner device is connected to a second splitting port of the second splitter/combiner device. The common port of the second splitter/combiner device is connected to the respective front port of the N×N AWG. In this way, a selected ONU may create a star link channel signal which is supplied to the respective back port of the N×N AWG at a predetermined channel wavelength, the predetermined wavelength being chosen in such a way that the star link channel signal is rooted from the respective back port to the front port that is connected to the back port at the predetermined wavelength. The first and second splitter/combiner devices supply the respective star link channel signal to the front port of the N×N AWG which also receives the downstream WDM signal created by the OLT. The star link channel signal is then routed to the back port of the N×N AWG that is connected to this front port at the given wavelength. In this way, the star link channel signal at the respective wavelength is supplied to the ONU which is connected to the respective back port of the N×N AWG. In this way, a selected ONU is able to transmit a star link channel signal to a selected further ONU by appropriately choosing the channel wavelength of the star link channel signal.
In order to create an upstream channel signal for the OLT traffic and an additional star link channel signal, two separate optical sources, especially lasers, may be provided within an OLT. Likewise, two photodetectors may be provided for receiving the downstream channel signal and a further star link channel signal.
In order to avoid collisions between downstream and upstream channel signals, EP 2 475 121 A1 suggests using a different band or order of the cyclic N×N AWG for the star link channel signals.
According to the communication system and method disclosed in EP 2 475 121 A1, each star link channel signal encounters the attenuation introduced by the two additional splitter/combiner devices. Thus, especially for a low-budget communication system that uses an N×N AWG having a higher number N of front and back ports, the attenuation caused by the first splitter/combiner device becomes inacceptable.